1. Field of the Invention
The present invention relates to a photo-coupling device for detecting what is called a photo-interrupter, in a non-contact manner, the presence or absence of an object passing through an object moving path.
2. Description of the Related Art
An example of previously known photo-coupling devices is disclosed in Japanese Utility Model Application Serial No. 1990-105449 filed on Oct. 5, 1990 by the inventor of the present invention. The photo-coupling device disclosed in this application has such a structure as shown in FIGS. 1 and 2 described later. Specifically, a light emitting section 1 and a light receiving section 2 are arranged to be opposed with respect to an object moving path X intervening therebetween. The light emitting section 1 is a light emitting device (e.g., light emitting diode) molded with a light-permeating or transparent material whereas the light receiving section is a light receiving device (e.g., a photo-transistor) molded with the transparent material. These sections are housed in a case 3 made of an opaque material. A light-projecting window 4b and a light-gathering window 4a are provided at the opposite surfaces 3a and 3b of the light emitting section 1 and light receiving section 2, respectively. In order to increase light detection precision (resolution), a light shading plate 6 provided with a slit 5 is bonded to the surface 3b of the light gathering window 4b in the case 3 with a boding agent.
Generally, the slit 5 in the light shading plate 6 is bored by etching or stamping techniques.
In the photo-coupling device thus constructed, the light emitted from the light emitting section 1 toward the light receiving section 2 goes across the object moving path X via the light-projecting window 4a. Hence, the presence or absence of the light reaching the light receiving section 2 is determined by the presence or absence of the object Y passing through the object moving path X. As a result, the presence or absence of the object can be detected in accordance with the presence or absence of the light on the light receiving section 2.
The photo-coupling device having the above structure, however, provides the following defects.
(1) As described above, the light shading plate 6 provided with the slit 5 is attached to the case 3 in order to increase the accuracy of detecting the object. In this case, since the bonding agent 7 is used to attach the light shading plate 6 to the case 3, if any shock is added to the case 3, the light shading plate 6 will peel off the case 3.
(2) It is technically difficult to bond and process the light shading plate using the bonding agent. The bonding strength and the attaching state depend on the manner of dealing with the bonding agent. This gives rise to inconveniences such as a long working time and high production cost.
(3) Where the light emitting section and the light receiving section are provided with a light-projecting window 4a and a light-gathering window 4b at their opposed case-wall surfaces and the slit is provided on the window 4a or 4b, there is a relatively long distance between the light shading plate and the light emitting section or light receiving section. Thus, if the aperture width of the slit 5 is made narrow in order to increase detection accuracy of the light passing through the light diaphragm, the signal-to-noise ratio will decrease owing to interference among passing light beams in a light receiving system in which plural light diaphragm windows are arranged. This becomes also an obstacle in miniaturizing the photo-coupling device.